Figure 2.

Schematic illustration of the energy levels involved in the charge transfer (CT) mechanism of the surface-enhanced Raman scattering (SERS) enhancement; hν—photon energy, E_F—Fermi level energy, E_CT—energies of occupied and unoccupied levels of the adsorbed molecules. (a) The electrons at the Fermi level of the metal are virtually excited into unoccupied molecular orbitals of the adsorbed molecule and back to the metal, (b) the electrons at the highest occupied molecular orbital can be virtually excited into the Fermi level of the metal and back to the adsorbed molecule. Reproduced with permission [7]. Copyright, 2009, Elsevier.